1. Field of the Invention
The subject invention relates to tunable RF networks and, more particularly, to apparatus for rapidly switching selected inductors in and out of a series network of such inductors.
2. Description of Related Art
The need has arisen for a variable RF inductor having no moving parts for use in an antenna matching network for matching signals in the 2 to 30 MHz HF range and subject to RF voltages on the order of three kilo-volts. It is further desirable that such an inductor be controllable by binary switching signals compatible with computer control at high speed. Theoretically, such an inductor may be constructed of a series connection of binary incremental inductor values of the required range and resolution with shorting switches used to remove unwanted inductive elements. Such switches must have low "ON" resistance and low "OFF" capacitance to avoid detuning of the RF circuit and power losses. PIN diodes meet these requirements but need a DC forward bias circuit to provide DC current for turn-on and a DC back bias voltage for turn-off.
Providing the appropriate switching signals for turning the PIN diodes "on" and "off" is not a straightforward matter because the DC bias circuit is also a path for shunting RF currents to ground. The resulting capacitive and resistive loading also causes unwanted RF losses and detuning of the RF network. An RF choke placed in each of the DC bias lines to the diode switches could theoretically provide the required RF isolation; however, the design of an RF choke that maintains a high impedance over the 2 to 30 MHz HF radio band is not practical. In addition, with voltages on the order of 3 kilovolts applied to the RF choke, the choke must have a very high impedance to minimize RF losses due to RF currents through its lossy elements.
In order to eliminate the foregoing problems, other workers in this field have proposed constructing the inductor windings using a tubular conductor with the control wires for PIN switches inside. The control wires then emerge from holes in the wall of the tubing where needed to control the PIN switch. Although conceptually simple, this construction is difficult to make. Extracting numerous control wires through the tubing and bringing them out small holes in the tubing wall is not a practical task in production. Thus, paralleling of multiple control wires with the inductor and choke windings is physically difficult to implement in a producible form.